Chapter 7: Current Sailing

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Chapter 7 Current Sailing

• Instructor Bob Garrison

All Questions and Problems from the Study Guide
PowerPoint Presentation by Forrest Meiere
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S7-1. The process of allowing for current in
determining the predicted course made good, or in
determining the effect of a current on the
direction of motion of a vessel is termed
______________.
Current Sailing
S7-2. The term current as used in this context
refers to three elements
1. the horizontal motion of water over the
ground, 2. the effect of wind and seas and 3.
other effects such as steering error, compass
error, speed curve error, tachometer error,
fouled bottom.
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S7-3. Current is not used in preparing a DR plot
however, a DR position can be corrected for
current. This position is termed an
_________________.
estimated position
S7-4. The estimated current is defined as the
current _____
developed from evaluation of known or predicted
forces using calculations, current tables,
diagrams and/or charts.
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S7-5. The actual current is defined as the
current _____
measured as the difference between the vessels
actual position (fix) and the position predicted
without current (DR position) .
S7-6. The set of a current is the _______________

direction toward which a current flows. Set
is expressed as True.
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magnitude or speed of the current. Drift
is expressed in knots or statute miles per hour
as appropriate..
S7-7. The drift of the current is the
________________
S7-8. The current drift angle is the
_________________

difference between course steered and the
estimated track or CMG
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S7-9. The difference between the track (COA) and
the CMG is that the track refers to an ________
or ________ direction of travel with respect to
the ground, whereas the CMG refers to the ______
direction of travel with respect to the ground.
intended expected
actual
S7-10. In determining the estimated actual
current from the vessel's DR position, the
current vector is drawn __________________________
_.
from the DR position to the fix
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S7-11. Ideally, the actual current is determined
by comparing the vessel's actual position with a
contemporaneous DR position. However, if no fix
is available, an ________________ may be
substituted for the fix. Such estimates, however,
are likely to be less accurate than if a fix were
available.
estimated position
S7-12. The current correction angle is a function
of the ___________________________________________
_
relative bearing of the set compared to the
course steered and the ratio of the drift to the
vessels speed.
End of Questions
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PROBLEMS
P7-l. If the course sailed was C000/S10.0, and a
FIX after one hour showed the course made good to
be CMG000/SMG7.0, what were the SET and DRIFT of
the current? Is the current fair or foul?
a. SET ________ b. DRIFT ________ c. (fair) /
(foul)
a. SET 180 b. DRIFT 3.0 kts c.
foul
P7-2. If the course sailed was C090/S6.0, and a
FIX after one hour showed the course made good to
be CMG092/SMG5.2, what were the SET and DRIFT of
the current?
a. SET ________ b. DRIFT ________
a. SET 257 b. DRIFT 0.8 kts
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P7-3. Given the course and speed to be
C060/Sl0.0, and the course and speed made good to
be CMG045/SMG9.0, what are the SET and DRIFT of
the current?
a. SET ________ b. DRIFT ________
a. SET 301 b. DRIFT 2.7 kts
P7-4. Given the course and speed to be C330/S9.5,
and the course and speed made good to be .
CMG3l5/SMG9.l, what are the SET and DRIFT of the
current?
a. SET ________ b. DRIFT ________
a. SET 223 b. DRIFT 2.5 kts
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P7-5. Given the course and speed to be C2l5/S8.0,
and the course and speed made good to be
CMG2l0/SMG7.3, what are the SET and DRIFT of the
current?
a. SET 076 b. DRIFT 1.0 kts
a. SET ________ b. DRIFT ________
P7-6. Given the course and speed to be Cl05/S8.5,
and the set and drift to be SET240/DRlFT3.0, what
are the course and speed made good?
a. CMG ________ b. SMG ________
a. CMG 123 b. SMG 6.7 kts
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P7- 7. Given the course and speed to be
Cl05/S8.5, and the set and drift to be
SET320/DRlFTl.4, what are the course and speed
made good?
a. CMG ________ b. SMG ________
a. CMG 099 b. SMG 7.4 kts
P7-8. Given the course and speed to be C150/S7.6,
and the set and drift to be SET024/DRlFf1.2, what
are the course and speed made good?
a. CMG ________ b. SMG ________
a. CMG 142 b. SMG 7.0 kts
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P7-9. After determining the current vector to be
SET000/DRlFT1.5, what course would you steer at a
speed of S8.0 to make good an intended track,
TR270? What would your SOA be? The time is now
1210, what is your ETA at a point along your TR
that is 10.3 miles distant?
a. COURSE _____ b. SOA _______ c. ETA ______
a. COURSE 259 b. SOA 7.0 kts c. ETA
1328
P7-10. After determining the current vector to be
SET215/DRlFT2.0, what course would you steer at a
speed of S10.0 to make good an intended track,
TR083? What would your SOA be? What is your ETE
to a point 4.3 miles distant?
a. COURSE _____ b. SOA _______ c. ETA
______
a. COURSE 074 b. SOA 8.6 kts c. ETA
30 mins.
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P7 -11. After determining the current vector to
be SET140/DRlFT2.0, what course would you steer
at a speed of S10.0 to make good an intended
track of TR020? What would your SOA be?
a. COURSE ______ b. SOA ______
a. COURSE 010 b. SOA 8.8 kts
P7-12. While running a course of C060 at a speed
of 10.0 kts for 1 hour and 10 minutes, you
actually cover a distance of 9.0 M with a CMG of
045. What are the SET and DRIFT of the current?
a. SET _____ b. DRIFT ______
a. SET 278 b. DRIFT 3.3 kts
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P7-13. While running a course ofC282 at a speed
of 10.0 kts for one hour, you actually cover a
distance of 7.5 M with a CMG of 294. What are the
SET and DRIFT of the current?
a. SET 072 b. DRIFT 3.1 kts
a. SET ______ b. DRIFT ________
P7-14. While running a course of C060 at a speed
of 14.0 kts for one hour, you actually cover a
distance of 15.6 M with a CMG of 063. What are
the SET and DRIFT of the current?
a. SET ______ b. DRIFT ______
a. SET 087 b. DRIFT 1.8 kts
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P7-l5. You are in Auxiliary vessel 273007
traveling in a generally southerly direction at
the west end of Vineyard Sound. Using a
hand-bearing compass, a magnetic bearing of 177M
degrees is taken on the light on Gay Head at
1320. A magnetic bearing of 284M is taken on the
Buzzards Bay entrance light at this same time.
Plot and label the fix. (Continued on next
slide.)
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P7-l5. (Continued A fix was plotted at 1320.)
You steer a course of 197 (true) and maintain a
speed of 8.0 knots. At 1400 the light on Gay Head
bears 074 M, and the Buzzards Bay light bears 331
M. What are
a. The coordinates of the 1400 fix? L
____________ Lo ___________ b. CMG ______
SMG ________? c. SET ______ DRIFT ________
of the current?
41 18.5 N 070 55.3 W
208 9.2 kts
257 2.1 kts
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(Use a variation of 15W in this and following
problems.) P7-l6. Assuming the results
determined in the answers to P7-l5 above, suppose
that you wished to voyage directly to the
Buzzards Bay entrance light and maintain a speed
through the water of 8.0 knots. What are the a.
True bearing and distance to the Buzzards Bay
entrance light?
BEARING _____ DISTANCE ________
BEARING 316 DISTANCE 7.3 NM
b. Course to steer, and estimated SOA?
COURSE ______ SOA _______ c. ETA at the
light? ______
C329 8.9 kts
1449
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P7-l7. You are sailing in a loran-equipped yacht,
Business. (This way, when your secretary receives
a call, she can legitimately say that you are
away on Business!) At 0930 you note the latitude
and longitude readouts on the loran as L 41
20.0' N and Lo 71 10.0'W. You steer a course
directly for buoy BW "SR" Mo (A) Whistle at the
entrance to the Sakonnet River, and maintain a
speed of 6.0 knots. (Continued on next slide)
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P7-l7. (Continued) After a few minutes it is
apparent that you are drifting slightly to the
left of the course to the "SR" buoy on your
initial heading. At 1020 you again take a loran
fix of L4l 23.2'N and Lo7l l2.7W'. What are
the a. CMG _____ and SMG _______?
b. SET ______ and DRIFT _______?
327 4.6 kts
186 1.7 kts
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P7-18. You are in a twin-screw sport fisherman,
Argo, south of Nomans Land. At 1700 you read the
loran and note the latitude and longitude as L
41 10.0 N, and Lo 70 50.0 W respectively.
You maintain a course of 050 (true) and speed of
12.0 knots. Meanwhile, someone inadvertently
pours coffee over the loran and it quits. At 1730
you take a bearing on the radar tower on Martha's
Vineyard with a hand-bearing compass, obtaining a
reading of 005 M. a. What is your 1730
EP? L ___________, Lo ____________
(Continued on next slide)
41 14.1 ' N 70 42.4' W
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P7-18. (Continued) b. Using the EP just derived,
the current set and drift are estimated to be
SET _____ DRIFT _______ c. To check your
position, you also take a bearing on the light at
Gay Head, which bears 332M. Plot the fix. A
revised estimate of the actual current is SET
_____ DRIFT ______ Is this likely to be a
better estimate? __________
080 2.2 kts
048 2.6 kts.
Probably
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P7-19. During a westerly voyage through Buzzards
Bay in the trawler. Equivoca, you estimate the
current set as 060, and the drift as 1.1 knots.
This checks (more or less) with the predictions
in the Tidal Current Chart Narragansett Bay to
Nantucket Sound. Now, at 1400, you pass close
abeam the Buzzards Bay entrance light enroute to
the Sakonnet River, maintaining 6.0 knots. The
planned or intended track is direct to buoy BW
"SR" Mo (A) Whistle at the entrance to the
Sakonnet River.
a. What is the intended track? _____
287
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P7-19. (Continued) b. Assuming that the current
stays the same as determined above, what are the
course and estimated SOA on this leg to reach
buoy BW "SR"? COURSE ____ SOA _________ c.
Layout the DR plot. What are the coordinates of
the 1500 DR position? L ___________, Lo
________
279 5.2 kts
41 24.8 ' N 71 09.8' W
d. At 1500, you take a bearing with a handheld
compass on the Tower (Aband LT HO) just southwest
of Sakonnet Pt, and read 320 M, and another
bearing on the tower at the end of Gooseberry
Neck, which reads 073 M. What is your estimate of
the current? SET _____ DRIFT ______
020 1.1 kts
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P7-20.At 1500, while on a compass heading of 075
C (use the deviation table given in Chapter 2 of
the ACN text) Auxiliary vessel, 273007, observes
Pt. Judith Light (actually, the tip of Pt.
Judith) on radar at a range of 2.0 nautical miles
and bearing 235R. After taking this fix, the
vessel turns to a compass course of 090C, and
maintains a speed of 5.0 knots. According to
available current information, the likely set is
000, and the drift 2.0 knots. At 1545, you take a
bearing on the Brenton Reef Light (Gp PI (2) 10
sec 87 ft 25 M) using a hand-bearing compass
(assumed to be free of deviation) and note a
bearing of 331 M. Plot the 1545 EP considering
both current and the 1545 LOP. What is the
distance from the 1545 EP to the Brenton Reef
Light? ________
2.9 NM
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P7-2l.A vessel cruising in an unknown current
with drift, Dft (knots), maintains a DR plot and
fixes its position every T minutes. The error in
the DR position compared to the fix is clearly
(T)(Dft)/60 (nautical miles). a. What is the
formula for the area of uncertainty, A, in sq.
nautical miles? ______ b. If the vessel fixes
its position once every 60 minutes and the drift
is 2.5 knots, what is the possible area of
uncertainty of its DR position? ______ c.
Suppose the navigator wishes to ensure that the
area of uncertainty is no more than 4 square
nautical miles. What is the maximum time between
fixes to ensure this accuracy? __________
-4 2 2
A 8.727 x 10 (Dft) (T)
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19.63 NM
27 minutes